Application Notes: continued
where
Heat Sinks
R
R
R
QJC = the junctionÐtoÐcase thermal resistance,
QCS = the caseÐtoÐheatsink thermal resistance, and
QSA = the heatsinkÐtoÐambient thermal resistance.
A heat sink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air.
R
R
QJC appears in the package section of the data sheet. Like
QJA, it too is a function of package type. RQCS and RQSA
are functions of the package type, heatsink and the inter-
face between them. These values appear in heat sink data
sheets of heat sink manufacturers.
Each material in the heat flow path between the IC and the
outside environment will have a thermal resistance. Like
series electrical resistances, these resistances are summed
to determine the value of RQJA
.
R
QJA = RQJC + RQCS + RQSA
(3)
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
